1. Field of the Invention
The present invention relates generally to a visible light communication system that utilizes visible light communication. More particularly, the invention relates to a visible light communication system that uses an LED.
2. Description of the Related Art
In recent years, visible light communication technology has been developed, which utilizes the visible light emitted from a light-emitting element such as a light emitting diode (LED) (see, for example, Japanese Patent No. 3391204). More specifically, it is proposed that an LED be used as light source in a transmitter and be intermittently driven at high speed, thereby to transmitting traffic information or the like to vehicles such as automobiles running on the roads.
At present, LEDs find various uses as light sources in lighting, as red-light lamps and head lamps on automobiles, as backlights in displays, as light sources in flash lamps and signaling devices.
In the visible light communication, the LED serves as light source, performing a signal-transmitting function and an object-illuminating function. Therefore, the area in which the LED can achieve visible light communication is limited to the area illuminated by the LED. This characterizes the visible light communication that covers a limited area and that can be easily recognized visually. However, the visible light communication has a problem. It does not work, while the illuminating function is not being performed.
A communication apparatus for use in vehicles, which utilizes the visible light emitted from the tail lamp on the automobile, has been proposed (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-323766). This communication apparatus superposes the visible light emitted from the tail lamp with signal pulses. Superposed with the signal pulses, the visible light can represent the data representing the danger level. This data is supplied to the following automobile, which receives the visible light. In this visible light communication, however, the data is transmitted only while the tail lamp remains on. While the tail lamp is off, no danger level data can be transmitted.
Such a problem arises also in visible light communication using the light coming from car head lamps or outdoor lamps. That is, the communication does not work at daylight. To solve this problem, the lamps may be kept on all time. Turning on the lamps at all times, whether necessary or not, results in another problem such as an environmental one. In the visible light communication using the car tail lamps, the tail lamps cannot be kept on all time. This is because their inherent function is to emit light to indicate that the brake is working, and to emit no light to indicate that the brake is not working.
Thus, the visible light communication available at present is performed in only a place or condition where the lamp should be always on or where the communication is unnecessary for a specific period (when the lamp is off). The lamps in any tunnel should be always on to enable the traffic information guide system to work, which employs the visible light communication. That is, the lamps are kept on, day and night, thus accomplishing visible light communication. The visible light communication is unnecessary for a specific period in, for example, an indoor system for giving people information. This is because this system is based on the assumption that the lamps remain on as long as anyone stays in the place. In other words, the visible light communication need not be achieved when no information recipients exist in the place.
Visible light communication apparatuses have been proposed, which can control the lighting by means of a pulse-width modulation (PWM) circuit (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-325085). Any visible light communication apparatus of this type can adjust the luminance of the light source, gradually and continuously, not step by step. Having the function of controlling the lighting, the apparatus efficiently works if used in, particularly, living rooms or shops where the atmosphere is important to people.
The visible light communication system should work, no matter whether the lamps remain on or off. This is not easy to accomplish, however. As already pointed out, the system cannot easily be provided because it needs to perform not only communication, but also the primary function of illuminating objects. (The primary function includes the function of turning the tail lamp on and off to show that the brake is working and not working, respectively). A prior-art visible light communication apparatus has been proposed, which has a lighting control function. For anyone who utilizes ordinary lighting function only, the apparatus may be hard to control and may be too expensive. As an ordinary light apparatus, it only need to assume only two states (i.e., on-state and off-state) or only three states (i.e., high-luminance on-state, and low-luminance on-state and off-state).